The present invention relates to a dioptric element array substrate (microlens array substrate, for example) having dioptric elements (lenses and prisms, for example) arranged two-dimensionally, and an image display device and an image display apparatus including such a dioptric element array substrate.
In general, in non-light emitting image display apparatuses, the light transmittance (or reflectance) of each of a plurality of pixels is changed with a drive signal, to modulate the intensity of light entering an image display device (display panel) to thereby realize display of an image and a character. Such image display apparatuses include a direct view mode (direct-view image display apparatus) and a projection mode, such as a projector, in which an image and a character are magnified and projected on a screen with a projection optical system (projection image display apparatus).
As the projector (including both front and back projection types), there are a single-panel projector using a color display panel having an array of red (R), green (G) and blue (B) color filters as the image display device, and a three-panel projector using three black-and-white display panels as the image display device.
Examples of non-light emitting image display devices include liquid crystal display devices, electrochromic display devices and electrophoretic display devices. Among others, liquid crystal display devices are broadly used for monitors, PDAs, mobile phones and projectors.
In a liquid crystal display device, a drive voltage corresponding to an image signal is applied to each of pixel electrodes arranged regularly in a matrix to change the optical characteristic of the liquid crystal layer in each pixel portion, to thereby realize display of an image, a character and the like. As the method for applying independent drive voltages to pixel electrodes, there are known a simple matrix method and an active matrix method in which nonlinear two-terminal elements or three-terminal elements are provided in the liquid crystal display device.
In the active matrix method, the device includes switching elements such as metal-insulator-metal (MIM) elements and thin film transistor (TFT) elements and interconnections for supply of drive voltages to the pixel electrodes. If intense light is incident on such switching elements, the resistance of the elements in the OFF state will decrease, resulting in discharging of charges obtained during voltage application. Moreover, no normal drive voltage will be applied to portions of the liquid crystal layer corresponding to the regions in which the switching elements and the interconnections are formed, causing failure in execution of the original display operation of the device. This will cause occurrence of light leakage in the black display state, for example, arising a problem of reduction in the contrast ratio of display.
In view of the above, in a transmission liquid crystal display device, for example, a shading layer called a black matrix is formed on a TFT substrate having the switching elements and the pixel electrodes formed thereon and on a counter substrate facing the TFT substrate via the liquid crystal layer, to shade light incident on the regions described above. Accordingly, in the transmission liquid crystal display device, with the shading with the black matrix, in addition to the TFTs, gate bus lines and source bus lines individually serving as light shields, the area of the effective pixel portion in each pixel, that is, the aperture ratio decreases.
Reduction of the sizes of such switching elements and interconnections to less than certain levels is difficult due to the limitations in their electric performances, fabrication technology and the like. Therefore, as the pitch of the pixel electrodes becomes smaller with achievement of higher definition and smaller size of liquid crystal display devices, the aperture ratio further decreases.
With the decrease of the aperture ratio, the amount of light transmitting through the liquid crystal display device decreases. This causes insufficient brightness for a liquid crystal projector that projects a smaller panel on a larger screen by magnification. In particular, in a single-panel projector, the brightness greatly decreases due to light absorption by color filters.
As one method for solving the above problem, there has been implemented a method in which microlenses are provided to converge light to individual pixel portions of a liquid crystal display device, to thereby improve the effective aperture ratio of the liquid crystal display device.
Japanese Laid-open Patent Publication No. 4-60538 (Literature 1) discloses a method as follows. White light is allowed to enter dichroic mirrors arranged in a fan shape to be divided into R, G and B color beams. The resultant color beams are allowed to enter microlenses, placed on the side of a liquid crystal display device closer to the light source, at different angles, to enable the respective color beams to be converged to the corresponding pixels. By adopting this method, the effect of improving the effective aperture ratio with the microlenses can be obtained, and in addition occurrence of light absorption by color filters can be prevented.
Japanese Laid-Open Patent Publication No. 7-181487 (Literature 2) discloses a microlens array substrate having a two-layered microlens array as shown in FIG. 11. The microlens array substrate includes a structure of a first microlens array 72, a middle glass plate 73 and a second microlens array 74 sandwiched between a pair of grass substrates 71 and 75. In this array substrate, R, G and B beams are converged to enter corresponding pixel open portions with the first microlens array 72. The principal rays of the B, G and B beams are then collimated with the second microlens array 74. Thus, with the first and second microlens arrays, the amount of eclipse at the projection lens can be reduced, to further improve the brightness.
Such microlenses, which are formed inside the counter substrate of a liquid crystal display device in most cases, are sandwiched between two glass plates so that light is refracted at the interface between a glass plate and a resin layer or between two kinds of resin layers to thereby obtain the light converging effect.
Two typical methods for producing a conventional microlens array substrate will be described with reference to FIGS. 12A and 12B.
In the first production method, a microlens array substrate is produced by following the steps diagrammatically shown in FIG. 12A.
A photoresist layer on a grass substrate is patterned. (2) The patterned photoresist layer is heated to cause running of the material by heating to thereby form a resist layer having a shape of microlenses. (3) The glass substrate is subjected to dry etching together with the microlens-shaped resist layer to impart the shape of the resist layer to the glass substrate (etch-back), to thereby obtain the glass substrate with a microlens array formed on the surface. (4) A cover glass plate is bonded to the resultant substrate via an adhesive layer, and the surface of the cover glass plate is grinded, to thereby obtain a microlens array substrate. An electrode, an alignment film and the like may be formed as required.
In the second fabrication method, a microlens array substrate is fabricated by following the steps diagrammatically shown in FIG. 12B.
A photoresist layer on a glass substrate is patterned by electron beam exposure, for example, to form a resist layer having a shape of microlenses, which is to be used as a master. (2) Using the master, a metal stamper is prepared by plating, for example. (3) Using the metal stamper, the shape of microlenses is transferred to a glass substrate, to obtain a glass substrate with a microlens array formed on the surface. (4) A cover glass plate is bonded to the resultant substrate via an adhesive layer, and the surface of the cover glass plate is grinded, to thereby obtain a microlens array substrate.
The two-layered microlens array substrate disclosed in the Literature 2 has the following problem.
As described above with reference to FIGS. 12A and 12B, the microlens array substrate normally has a structure in which microlenses are formed on a glass substrate and a cover glass plate is bonded to the microlens-formed glass substrate, that is, a structure of microlenses being sandwiched between two glass substrates. The thickness of the cover glass plate is generally about 30 μm for a normal single-layered microlens array substrate.
For a two-layered microlens array substrate as disclosed in the Literature 2, the thickness of the cover glass plate (placed on the light outgoing side of the substrate) must be far smaller than 30 μm. The reason for this is as follows.
In the microlens array substrate of the Literature 2, microlenses having the same focal length are arranged in two layers from the standpoint of brightness. Accordingly, to ensure that the positions to which beams are converged with the first microlens array are very near the liquid crystal layer, the liquid crystal layer must exist just behind the second microlens array, and thus the cover glass plate placed between the second microlens array and the liquid crystal layer must be extremely thin.
It is however very difficult to extremely thin the cover glass plate. Even the thickness of the cover glass plate used for the conventional single-layered microlens array substrate is as small as about 30 μm. Such a thin cover glass plate is normally produced by first bonding a glass plate having a thickness sufficiently large compared with a desired thickness to the surface of the microlens array and then grinding the glass plate to the desired thickness.
A variation of about ±5 μm is normally produced in the thicknesses of glass plates even when the plates are of a high grade. Moreover, the grinding of the cover glass plate is made so that the total thickness from the surface of the cover glass plate to the base glass plate on which the first microlenses are formed reaches a predetermined thickness, not being made with checking of the thickness of the cover glass plate itself. Therefore, if a variation exists in the thicknesses of the base glass plates on which the first microlenses are formed, the grinding amount of the cover glass plate varies by this variation.
In the two-layered microlens array substrate, a middle glass plate is placed between the first and second microlens arrays. A variation is also produced in the middle glass plates as in the base glass plates. As in the cover glass plate described above, the middle glass plate is ground so that the total thickness reaches a desired thickness after being bonded to the substrate on which the first microlens array is formed. Therefore, only the variation in the base glass plates, if any, is reflected as the variation in the total thicknesses of the substrates obtained after the grinding of the middle glass plate.
In the production method described above, if the cover glass plate for the two-layered microlens array disclosed in the Literature 2 is extremely thinned, the cover glass plate may possibly be completely ground off due to the variation in the thicknesses of the base glass plates (about ±5 μm), resulting in damage to the second microlens array. If the thickness of the cover glass plate is set at 5 μm or more to prevent this problem, however, the beams will fail to be converged at positions very near the liquid crystal layer, that is, very near the light outgoing-side surface of the cover glass plate, resulting in low light use efficiency.
Etching of glass may be adopted as the method for thinning the cover glass plate. However, a variation in etching amounts of about ±5 μm to 10 μm also occurs in this method, and thus the same problem arises.
As described above, when the two-layered microlens array substrate of the Literature 2 is used for a liquid crystal display apparatus, the light use efficiency improves compared with the case of using the single-layered microlens array substrate of the Literature 1, but the display is not yet sufficiently bright. Moreover, the parallelism of the light outgoing from the microlens array substrate is not sufficiently high, and this results in insufficient color purity, for example. Further improvement in display quality is therefore requested.
Liquid crystal display devices were taken as an example in the above discussion on the problem related to the conventional two-layered microlens array. Occurrence of the above problem is not limited to liquid crystal display devices, but is common to microlens array substrates used for any non-light emitting display devices having display medium layers other than the liquid crystal layer. Also, the above problem occurs, not only in the microlens array substrate, but also in other dioptric element array substrates (microprism array substrate, for example) having dioptric elements arranged two-dimensionally.
In view of the above, an object of the present invention is providing an image display apparatus that is bright and excellent in display quality. Another object of the present invention is providing a dioptric element array substrate suitably used for such an image display apparatus.